One of the most successful systems today for producing sheet material products with multicolored or enhanced graphic images for signs and like displays is the GERBER EDGE.TM., manufactured by Gerber Scientific Products, Inc. of Manchester, Conn. The GERBER EDGE.TM. is typically used to print vinyl graphics for signs or like displays, wherein multicolored or enhanced graphic images are printed on a vinyl sheet, and the sheet is cut along the periphery of the graphic images to create a sign or like display. The system uses a thermal print head to print the graphic images on the sheet, and a cutter to cut the sheet along a peripheral edge surrounding the graphic images. The print head and the cutter are controlled by a microprocessor having a common database so that the printed images and the cut edges correspond positionally in the final graphic product.
The vinyl sheet has a series of feed holes along each marginal edge, and is driven over a roller platen mounted below the print head by drive sprockets engaging the feed holes. A removable cassette carrying a donor web bearing transfer ink is mounted adjacent to the print head so that the donor web is interposed between the print head and the vinyl sheet. Heating elements of the print head are selectively energized to transfer ink from the donor web to the vinyl sheet in accordance with commands from the microprocessor to create graphic images on the vinyl sheet. Each cassette carries a donor web bearing a single color of transfer ink, and the cassettes are interchanged to create multicolored images, different shades and/or colors. The drive sprockets and vinyl sheet are slewed back and forth during printing operations to apply the different color transfer inks.
The GERBER EDGE.TM. system is described in U.S. Pat. No. 5,537,135 entitled "Method And Apparatus For Making A Graphic Product", which is assigned to the Assignee of the present invention, and is hereby expressly incorporated by reference as part of the present disclosure.
A typical ink web is comprised of a resin and/or wax layer containing the transfer ink, a release layer superimposed over the resin/wax layer, a carrier layer superimposed over the release layer, and a back coat superimposed over the carrier layer to provide a low-friction surface for engaging the print head. When the heating elements of the thermal print head are energized, the portions of the resin layer contiguous to the print head undergo transformation from (i) a solid state, to (ii) a semifluid or viscous state, and at the highest temperatures, to (iii) a less viscous, liquid state. Then, as the heating elements are de-energized and upon passage of the ink web and vinyl sheet beyond the print head, the heated portions cool down and return from the liquid, semifluid or viscous states to the solid state, as they approach ambient temperature.
During these changes in physical states, the coefficients of friction and thus the forces transmitted between the vinyl sheet, ink web and print head vary, which leads to variations in the surface velocity of the vinyl, and can in turn cause sagging or like deformation in the vinyl as it passes beneath the print head. Typically, the longer the print head (i.e., the dimension of the print head in the axial direction of the platen), the greater are the variations in the forces applied to the vinyl. Because the vinyl sheet is flexible, the increase in the forces transmitted between the vinyl and print head on each cool-down cycle can cause a lag or positional error between the portions of the vinyl contiguous to the print head and other areas, such as the marginal portions of the vinyl engaging the sprockets. These variations in the vinyl velocity and positional errors lead to fluctuations in image intensity and, concomitantly, a degradation in print quality.
Moreover, because the sprockets are used to positively drive the vinyl, the increase in forces transmitted between the print head and vinyl, causes an increase in the forces transmitted between the sprockets and vinyl, which can in turn lead to deformation of the vinyl's feed holes. Hole deformation can cause shifting of the vinyl, which in turn affects the registration of the image on the vinyl and leads to a degradation in print quality, e.g., color shifts. The registration may change from one pass of the vinyl sheet beneath the print head to the next (pass-to-pass registration) and/or may change upon transfer of the vinyl from the printing apparatus to the vinyl cutting apparatus (print-to-cut registration). Hole deformation typically increases with repeated passes of the vinyl beneath the print head. In addition, the greater the duty cycle in energizing the print head, the more noticeable are these negative effects on print quality.
It is an object of the present invention to overcome the drawbacks and disadvantages of prior art apparatus for making graphic products on sheet material.